In conventional passive radiator speaker systems for reproduction of a low-frequency sound, a single passive radiator is generally used along with a drive speaker unit in a cabinet. However, to reduce a distortion within a low-frequency sound range, a method of using two passive radiators has been proposed (see, for example, see Patent Document 1).
FIG. 21 is a cross-sectional view illustrating a structure of a major portion of a speaker system disclosed in Patent Document 1. In FIG. 21, 1 indicates a cabinet, 2 indicates a drive speaker unit attached inside the cabinet 1, 3 indicates a first passive radiator attached to the cabinet 1, 4 indicates a second passive radiator attached to the cabinet 1, facing the first passive radiator, and 5 indicates an internal space of the cabinet 1 to which the first passive radiator 3 and the second passive radiator 4 are attached.
An operation of the thus-configured speaker system will be described. When an electrical signal is applied to the drive speaker unit 2, a sound is radiated from the diaphragm of the drive speaker unit 2. The pressure of the radiated sound vibrates the first passive radiator 3 and the second passive radiator 4 acoustically coupled via the space 5, so that a sound is radiated to an external space outside the cabinet 1. In this case, since the first passive radiator 3 and the second passive radiator 4 are attached to surfaces facing each other of the cabinet 1, the vibration of the cabinet 1 caused by the vibration of the first passive radiator 3 and the vibration of the cabinet 1 caused by the vibration of the second passive radiator 4 are canceled with each other, so that the vibration of the cabinet 1 caused by the vibration of the passive radiator can be prevented.
Since passive radiators do not have a drive system including a voice coil, a magnetic circuit and the like, unlike speaker units, the nonlinearity of the support system would be a major cause of the occurrence of a distortion. In the design of speaker units, consideration needs to be given so as to stabilize the vibration of the voice coil so that the voice coil is prevented from contacting and damaging a magnetic gap of the magnetic circuit. In the design of passive radiators, a drive system including a voice coil, a magnetic circuit and the like is not possessed, and therefore, such consideration is not required, and only consideration is given so as to improve the linearity of a supporting force of the supporting system. Also, the limit of low-frequency sound reproduction of a passive radiator is determined by the acoustic antiresonance of the weight of the vibration system of the passive radiator and the air in the cabinet. In this case, the stiffness of the supporting system of the passive radiator needs to be sufficiently small as compared to the stiffness of the air in the cabinet.
The supporting system of a passive radiator includes a damper supporting a center portion of a diaphragm and an edge supporting an outer circumferential portion of the diaphragm. The edge may have various shapes. The most widely used edge is a roll edge which has a semicircular cross-section. When the roll edge is employed, the diaphragm can be supported while keeping the linearity of the supporting force even if the diaphragm is vibrated with a large amplitude. Note that, as disclosed in Patent Document 2, the roll-shaped edge has different displacements of air in the vertical amplitude direction, and therefore, even if the supporting force is linear, a sound radiated from the edge includes a distortion component.
In the speaker system described in Patent Document 1 above, as illustrated in FIG. 21, the front-to-rear direction with respect to the radiation direction of a sound of the first passive radiator 3 is opposite to the front-to-rear direction with respect to the radiation direction of a sound of the second passive radiator 4. In other words, while the first passive radiator 3 radiates a sound from the front surface to the external space, the second passive radiator 4 radiates a sound from the rear surface to the external space. As a result, an asymmetrical distortion included in the sound radiated to the external space from the first passive radiator 3 and an asymmetrical distortion included in the sound radiated to the external space from the second passive radiator 4 are canceled with each other, so that the distortion caused by the roll shape of the edge as described above is reduced.    Patent Document 1: Japanese Laid-Open Patent Publication No. 8-79876    Patent Document 2: Japanese Laid-Open Patent Publication No. 5-260581